Recently, wireless communication systems owners and operators have asked communication system designers to increase the capacity of wireless communication systems. This capacity, or the ability to carry user data, is what the operators sell to the system users. Therefore, increased capacity means increased profitability for communication system operators.
One method of improving capacity is to reduce the amount of energy needed to transmit a bit of information over the wireless air interface. Several methods of reducing the energy per bit have been proposed. Two of these proposals include transmitting user data using adaptive antenna arrays, and transmitting data using a method of transmission known as "orthogonal transmit diversity."
Transmitting user data with adaptive antenna arrays is a technique implemented by measuring the channel characteristics and modifying the gain and phase of signals applied to each element in an antenna array in order to create an antenna pattern that maximizes the power delivered to the subscriber unit. One of the disadvantages of adaptive array technology is the need for constant measurement and feedback of the channel characteristics and the subsequent recalculation of the adaptive array weights used to modify signals for each antenna element. The time needed to measure and compute the weights limits the speed at which the antenna pattern may be modified to compensate for a changing channel. Thus, when the subscriber travels at a higher speed, the channel changes at a rate higher than the rate of compensation in the adaptive antenna array. Thus, the feedback loop in the adaptive array technique cannot keep up with a quickly changing channel between the base station antenna and a higher-speed subscriber unit.
With regard to orthogonal transmit diversity, it is implemented by first converting a serial traffic channel into two or more parallel traffic channels using a multiplexer. If the traffic channel is converted into two parallel traffic channels, the two parallel traffic channels operate at one half the rate of the serial traffic channel input into the multiplexer.
Once the data is multiplexed, each parallel traffic channel is spread with a different spreading code to produce two or more spread traffic signals. These two spread traffic signals are then added together and transmitted from either a single antenna or separately transmitted from two or more separate antennas.
Some designers have proposed a communication system that switches from adaptive antenna transmission to orthogonal transmit diversity when the feedback required in adaptive antenna transmission begins to fail. A first problem with this mode switching proposal is that the receiver must be able to operate in two modes, where the adaptive array mode demodulates with one despreading code, and the orthogonal diversity mode demodulates with multiple codes despreading. Operating in two modes requires additional complexity in the receiver.
Because it is desirable to use a single receiver structure, it has been proposed that radio systems having orthogonal transmit diversity capabilities always transmit with multiple codes, even when transmit diversity is not active. Presently known transmitters using adaptive arrays presume that a single code spreads the transmitted data, which is a mode incompatible with the orthogonal transmit diversity proposal.
Another problem with the mode switching proposal is that the base station is required to send a message which tells the mobile which mode to use. Such messaging is slow in switching modes and requires additional mobile station circuitry or programming to implement.
Therefore, it should be apparent that there is a need for an improved method and system of transmitting and receiving a traffic channel using techniques from both orthogonal transmit diversity and adaptive antenna array transmission.